When first introduced, Koro Sensei was a jovial individual who demanded absurd and seemingly mundane requests despite claiming he would be the cause of the planet's destruction in a year. He was also rather kind to others in general and could switch from silly and childish to serious on the fly. According to Nagisa, Koro Sensei also was unintentionally petty at others at times and loved gossip, both traits heavily noted throughout the series.

Otaku Trivia:

1) Koro Sensei is usually represented by a octopus due to his appearance of his head and tentacles. Aware of this, he often draws octopus mascots when scoring exam marks and makes jokes about octopuses.

2) Koro Sensei is ambidextrous, being able to write/perform a task with either hands, and being able to work with both hands at the same time.